The present invention relates to a pump dispenser for dispensing accurate small quantities (doses) of medical or hygienic fluid substances, and in particular ophthalmological substances for treating the eye or for eye hygiene. The object to be achieved is that each time the device is actuated a quantity of substance equal to one drop is expelled from a receptacle, i.e. a volume of about 20 to 50 microliters (cubic millimiters) to within plus or minus about 8%. Even with such a wide tolerance, it is necessary to have a very accurate mmechanism when dispensing such a small quantity. The substance to be dispensed may be an oil, a gel, or a very fluid eye lotion. The device must therefore be capable of dispensing substances having very different viscosities. Since the dispenser device is generally sterile, it is important, except when the device is in use, to avoid contact between the substance and air in the vicinity of the expulsion orifice, in order to avoid polluting the substance. It is also necessary to avoid contact with metal parts such as springs in order to avoid chemical reactions. The device must be capable of being screwed to an aluminum tube or to a glass flask. Finally, it may be observed that such a device is discarded after use. It constitutes a kind of throwaway packaging and its manufacturing cost must therefore be relatively cheap.
Prior devices of the positive displacement type or of the metering valve type are unsuitable for meeting these conditions.